Great question! Unfortunately I'm not really sure.
In languages like German and Danish, etc, the indefinite article is implied when you're talking about a person and their profession: «Han er dommer. Min mor er advokat. Pigen er ingeniør.» However, every sentence I've seen that applies an attribute to a person includes the indefinite article: «Hun er en dreng. Det er en farverig fugl. En bamse er en legetøj.»
That being said, translation's not an exact science, but more of an art, and to most speakers of American English it's acceptable to say "He is a vegetarian," so I'd say that if it makes sense in your head to translate it that way, then translate it that way.
Are there any native speakers that can chime in?